In this paper, an improved technique of using open-ended rectangular waveguide sensor for simultaneous non-destructive complex electromagnetic parameters determination of compound absorbing materials is developed. The technique is based on employing a layered material medium, which consists of material to be tested and known materials, sandwiched between sensor aperture and perfect conductor plate to obtain two complex reflection coefficients necessary to achieve this purpose. Finite-Difference Time-Domain method is adapted to account for the finite flange size of the sensor and numerically calculate the sensor reflection coefficient under different test conditions. The technique can be employed for cases of single layer and multilayer medium planar sheet measurement. The parameters of the material under test are obtained by fitting both the calculated and measured reflection coefficients using the iterative optimization technique. The details related to the analysis, FDTD modeling and testing procedure of the proposed technique is discussed. The simulations and experimental results are compared with the published data by literatures and companies to validate the proposed technique. The technique is promising for potential applications such as design and fabrication of sheet or layered coating materials and nanocomposite material and bio-medium characterization.